This invention relates to a dispersing nozzle with variable throughput, in particular with continuously variable throughput. In addition, a coating unit and a spray gun equipped with this dispersing nozzle is described. The dispersing device is based on the principle of a jet disperser, and consists of at least one inlet for the material to be dispersed, and of a chamber with a multiplicity of openings arranged in rows or slots along the chamber wall, which lead into an outlet chamber, and with an outlet for the finally dispersed material; within the chamber there is a displaceably mounted piston which, depending on its position within the chamber, partially or completely shuts off a specific number of the openings or slots for the passage of the flow of dispersed material.
A number of different dispersing devices for the mixing and dispersion of, for example, oil-water emulsions of differing composition, have been disclosed. These devices have in common the principle of energy uptake in a dispersing gap or in appropriately shaped bores of the devices. Here the dispersed material is generally driven through the gaps or bores under increased pressure in order to produce a required range of particle sizes in the emulsion, depending on the differential pressure.
Two-component polyurethane coatings (2K PU coatings) are not mixed together until shortly prior to application, owing to the only limited processing time (pot life) of the coatings. This pot life can range from a few minutes to hours, depending on the reactivity of the coating systems. Whereas such two-component systems have in the past been used dissolved in organic solvents, more recently a wealth of water-dispersible two-component systems have been developed. The water-dispersible two-component systems typically consist of a hydroxyl-containing resin component (binder, polyol) and of a polyisocyanate component (curing agent, cross-linking agent). Here the hydroxyl-functional resin component is generally in the form of an aqueous dispersion, and the polyisocyanate component is a hundred-per-cent anhydrous component or is dissolved in a solvent. Such systems are known, for example, from the document EP-A 583 728. A disadvantage of these coating systems is that the well-known coating quality of the two-component systems based on pure organic solvents has not yet been achieved in some fields of application.
This applies primarily in fields of application in which particularly high optical proper ties and a high resistance are required.
It is known that coating dispersions having as small a particle size as possible should be used in order to achieve coating surfaces of high quality. For this reason, polyol dispersions having a sufficiently small particle size of less than 500 nm, preferably 10 to 200 nm, are generally used in aqueous two-component polyurethane coatings. The dispersion of the per se hydrophobic isocyanate component is not carried out until shortly before the application of the coatings, because the polyisocyanate component reacts with water and therefore has only a limited stability in storage in the presence of water. However, the dispersion of the per se hydrophobic isocyanate component in the aqueous hydroxyl-functional resin dispersion by conventional static mixing devices causes considerable difficulties. The reason is to be seen in the fact that, in the course of the emulsification, the isocyanate component becomes stabilised on the surface of the emulsion particles already formed, so that the superficial stabilising layer is an obstacle to a further dispersion. Consequently, aqueous two-component polyurethane coating emulsions typically exhibit a bimodal particle-size distribution, with a first distribution maximum having a particle size which corresponds to that of the hydroxyl-functional resin dispersion and a second distribution maximum having a particle size of above 10,000 nm (isocyanate component), considerable proportions with particle sizes of above 20,000 nm still being present.
Polyisocyanates hydrophilised by chemical modification and polyisocyanates containing external emulsifiers have already been developed. These render possible a significantly easier dispersion by static mixing devices to an average particle size of less than 1000 nm, but they produce cured coating films which are insufficiently stable for many fields of application. Coating films with good stability are only obtained, however, by using hydrophobic polyisocyanate components.
The concept that the dispersibility of the isocyanate component is restricted by the stabilisation reaction which takes place on the surfaces of particles already present has prompted a search for practicable ways of achieving as finely-divided a dispersion as possible within very short periods of time, within which no appreciable surface stabilisation has as yet taken place. In particular, a heating process which would accelerate the reaction of the polyisocyanate with water is also to be avoided during the dispersion.
European Patent EP 685 544 A1 describes a process for producing aqueous two-component polyurethane coating emulsions by mixing binder resins together with polyisocyanates and water, wherein the mixture is pressed, under a pressure of 1 to 30 MPa, through a dispersing nozzle constructed on the principle of a one-step or multistep jet disperser. Special bimodal coating emulsions are produced in this case.
To make it possible to vary the throughput through such a jet disperser, a variant of the jet disperser is equipped with a multiplicity of bores, which can be covered in succession by means of a displaceable inlet pipe in order to discretely adjust the throughput through the emulsifying device.
Here the proposed construction of the disperser has proved to be very unfavourable, as the displaceable inlet pipe is wholly immersed in the solution to be dispersed. In the case of a relatively long operation, for example, with coating emulsions, this can lead to unwanted deposits. The roller propulsion indicated for the inlet pipe is likewise unfavourable, as it forms unwanted dead spaces and allows the dispersed material to escape. It has also been found disadvantageous that this nozzle cannot be regulated sufficiently rapidly, for example, within seconds, which is necessary in order to produce a constant quality of emulsion in cases where the batch quantities fluctuate.
The object of the invention is to develop a dispersing device which does not have the above-mentioned disadvantages and nevertheless renders possible, in particular, a continuous variation of the quantitative throughput of the dispersed material, while the quality of the dispersion remains constant.
It has been found, for example, that the bodywork of automobiles can particularly advantageously be provided with coatings of very high quality, if the emulsification of the polyisocyanate in the aqueous polyol component is effected continuously by means of the dispersing nozzle according to the invention immediately prior to introduction into the spray gun or atomising cone of a coating unit. However, there are problems in using known dispersing devices if, owing to the geometry of the automobile bodywork, the take-up of the coating fluctuates over very short intervals of time.
Thus, a further object of the invention is to provide a mixer for high-quality aqueous two-component polyurethane coatings which continuously produces a constant quality of emulsion in cases where the batch quantities fluctuate.
Prevailing prior art provides spray guns which, owing to the complex design of their feed and mixing mechanisms, achieve only very short operating lives when used with coating systems containing abrasive fillers and subsequently have to be expensively cleaned, so that they are unsuitable in practice for rapidly reacting two-component coating systems containing fillers.
Accordingly, a further object of the invention is to render possible the direct processing of rapidly reacting coating systems and to integrate the dispersing device into a spray coating device (spray gun).
Surprisingly, this object is achieved by the following dispersing device described in more detail below.